1. Field of the Invention
The present invention relates to advanced television (ATV) systems; and particularly to a high definition television (HDTV) broadcast system and method utilizing properties of radiated electromagnetic waves and antenna technology to allow for compatibility with existing television receivers, and to contain the radio frequency (RF) bandwidth of broadcast HDTV signals within current television channel allocations.
2. Discussion of the Known Art
A. Current ATV Regulation Policy.
On Sept. 1, 1988, the Federal Communications Commission (FCC) released a Tentative Decision and Further Notice of Inquiry (NOI), FCC 88-288, with respect to an ongoing review of Advanced Television Systems and Their Impact on the Existing Television Broadcast Service (MM Docket No. 87-268). A summary of the NOI is printed in the Federal Register of Oct. 3, 1988, 53 FR 38747-49. As described in the NOI, current state of the art non-broadcast television techniques provide picture resolution and color approaching that obtainable with 35 mm film.
The FCC determined that the public would benefit from a terrestrial broadcast ATV service, but that most systems currently proposed by industry had one or more of the following disadvantages
1. Non-compatibility with existing color television receivers manufactured according to the United States 30 frame per second, two-field interlaced scan, 525 line NTSC (National Television System Committee) color standard, adopted in 1953.
2. A requirement of more than six megahertz bandwidth for transmission of the entire ATV signal, thus exceeding the currently allocated terrestrial television broadcast channel bandwidth if the proposed ATV signal is modulated and broadcast on a radio frequency carrier wave.
3. For those proposed ATV systems categorized as compatible with existing receivers, picture resolution is diminished when received on a standard television set, and/or the quality of the picture when reproduced on a "high definition" receiver is degraded during movement of the televised image.
The FCC initially concluded that any broadcast standards for a new ATV terrestrial service shall be such as to overcome (1) and (2) above. That is, an approved system would be one that is compatible with the many existing color television receivers now in use in the United States so as not to make them suddenly obsolete, and one that will not require additional broadcast frequency allocations to realize maximum picture definition.
The requirement that the existing broadcast television frequency allocations be used for an ATV broadcast service, is dictated both from an administrative and a technical point of view. First, any additional required spectrum might be at the expense of another (non-television) allocated service and would require lengthy hearings to obtain. Second, the additional spectrum might be so far removed in wavelength from existing television channel frequencies on which the "compatible" parts of ATV signals must be broadcast, that differences in propagation characteristics would likely degrade, rather than enhance, the received picture quality.
By way of a policy statement issued Mar. 21, 1990, the FCC redefined its objectives by stating its intention to select a so-called "simulcast" HDTV standard, viz., one that is "compatible with the current 6 MHz channelization plan but employing new design principles independent of NTSC technology". Statement of Chairman Alfred C. Sikes to the Advisory Committee for Advanced Television, 3/21/90. Thus, the FCC's initial conclusion that any newly adopted HDTV standard should be NTSC compatible, was relaxed in favor of allowing for a new television signal format, provided the RF spectrum of the broadcast signal does not exceed a 6 MHz bandwidth. See also First Report and Order, MM Docket No. 87-268, FCC 90-295, Adopted Aug. 24, 1990, and Released Sept. 21, 1990.
B. Patented Compatible ATV Techniques.
U.S. Pat. No. 4,521,803 issued June 4, 1985, discloses a system for compatible transmission of high-resolution television. Basically, a high resolution (e.g., 1,050 line) television camera simultaneously produces signals corresponding to a pair of adjacent scanned lines of an image. To provide a compatible 525 line picture for reception by existing receivers, luminance and chrominance signals for each scanned pair of adjacent lines are additively combined and broadcast at the conventional line frequency rate of about 15,750 lines per second. The luminance signals for each adjacent line pair are also subtracted from one another to produce difference signals that modulate a carrier phased in quadrature with the luminance carrier of the broadcast television signal. Suitable decoding equipment in a "high definition" television receiver would derive luminance signals for each of the 1,050 scanned lines by detecting and processing the additively combined and difference signals, according to the patent.
Another compatible HDTV system utilizing a high-resolution vidicon capable of 1,050 line resolution, is disclosed in U.S. Pat. No. 4,707,728 issued Nov. 17, 1987. According to the patent, a difference or "delta" signal representative of fine picture detail is transmitted during the vertical blanking interval, and a suitably equipped receiver detects and combines the delta signal with received limited bandwidth signals, to reproduce a high-definition picture. Maximum definition is obtainable only for still images at the receiver, however, since transmission of the delta signal is inhibited during frame-to-frame motion of the televised image.
European Patent 114,694 granted May 18, 1988, discloses a system for transmitting high resolution image line signals in a time multiplex manner, wherein chrominance information is followed by luminance information during corresponding portions of each line scan period (the so-called multiplex analog component or "MAC" system). Two conventional television channels are required for transmission and reception of a high resolution television picture, while a standard resolution picture may be received over one of the channels.
C. Principles of the Present Invention.
As far as is known, no system or technique has been proposed that utilizes physical properties of radiated electromagnetic waves and antenna technology, to enable broadcasting of HDTV signals compatible with existing television receivers and confined within the currently allocated spectrum for the terrestrial broadcast television service.
U.S. Pat. No. 2,820,844 issued Jan. 21, 1958, shows a black-and-white compatible color television broadcasting system, wherein brightness information for each image line is radiated from a horizontally polarized antenna, and corresponding color information for each line is radiated from a vertically polarized antenna.
It is known generally that if mutually orthogonally polarized transverse electromagnetic (TEM) waves, e.g., one horizontally polarized and the other vertically polarized, are transmitted in free space at the same frequency or wavelength, different information modulated on each of the waves can be separately received and demodulated by use of correspondingly polarized receiving antennas. See M. Javid & P. M. Brown, Field Analysis and Electromagnetics, at 294, McGraw-Hill (1963).
R. C. Johnson and H. Jasik, in their Antenna Engineering Handbook, McGraw-Hill (2d ed. 1984), point out (at page 23-9) that "[f]or any arbitrarily polarized antenna, there can be another antenna polarized so that it will not respond to the wave emanating from the first antenna. The polarizations of the two are said to be orthogonal."
Johnson and Jasik also disclose that the deployment of an increasing number of communications satellites has required use of the same frequency to communicate with adjacent areas on the earth. To enable such "frequency reuse", orthogonal polarizations are employed. Orthogonal circularly polarized transmitting and receiving antenna pairs, when used for frequency reuse with communications satellites, are reported to provide channel isolation in excess of 20 dB. Johnson & Jasik, at 23-4.
Current television transmission standards prescribed by the FCC at 47 C.F.R. 73.682 (a)(14) call for horizontal polarization as "standard", but allow circular or elliptical polarization to be employed if desired. In the latter case, the licensed effective radiated power (ERP) of the vertically polarized component may not exceed the licensed ERP of the horizontally polarized component.